This application is being concurrently filed with the following applications: "COMBINED MODULATOR SCHEME FOR SPATIAL LIGHT MODULATORS," Ser. No. 08/038,392; TI-17335; and "PROCESS AND ARCHITECTURE FOR DIGITAL MICROMIRROR PRINTER," Ser. No. 08/038,398; TI-17632.
1. Field of the Invention
This invention relates to printers using spatial light modulators (SLMs), more particularly to gray scale printing using SLMs.
2. Background of the Invention
Achieving gray scale with a spatial light modulator presents several difficulties. One occurs when using binary SLMs, whose elements can only be either ON or OFF. In printing systems, that means either the photosensitive media either receives light from that element or not. If it does not receive light, the drum does not pick up any toner, and the resulting page remains blank (white) at that spot. If the photosensitive media does receive light, the drum picks up toner and the resulting page becomes black at that spot. In most binary SLMs, there exists no way to expose a photosensitive media so as to pick up a lesser amount of toner in an analog-fashion.
Laser printing systems can achieve gray scale by modulating the light in an analog fashion, typically using the power of the laser. However, laser systems cannot normally modulate in this analog fashion fast enough to achieve more than a minimum number of gray levels. Laser scanning systems print pixel-by-pixel by scanning the laser beam along raster lines. For example, the user wants to print an 8.5" by 11" paper with 600 dots per inch (dpi) at 40 pages per minute. Each page requires 11".times.600 dpi, or 660 lines. At 40 pages per minute this equals 4400 lines per second, where each line has 8.5.times.600, or 5100 pixels. This requires 22.44.times.10.sup.6 pixels per second, allowing 44.6 nanoseconds per pixel. Lasers cannot modulate fast enough in an analog fashion to give gray-scale exposure during the short pixel time. Some SLMs have this capability.